idea for multitasking

[unknown user]

i was thinking about something for writing my kernel to multitask. what if i disassembled each prog i was going to run and ran the asm commands one by one? i asked this on EH but nobody answered.

[Tim]

No, don't (unless you're trying to write a Java VM). It would be veryveryveryvery slow. Assuming you're on the x86, you can do simple multitasking on the timer interrupt.

• Give each task (thread or whatever) its own kernel stack
• On every interrupt, store all registers on the stack (pusha and push each segment register).
• On the timer interrupt, choose which task will run next
• On the way out of every interrupt handler, switch to the current stack (mov esp, something)

For example:

Code: Select all

timer_handler:
pusha
push ds
push es
push fs
push gs

mov eax, KERNEL_DS
mov ds, eax
mov es, eax

; Centralised interrupt handler. Returns new ESP in EAX
call interrupt_hander
mov esp, eax

pop gs
pop fs
pop es
pop ds
popa
iret

[unknown user]

now how exactly would you put that into action. i'm new to asm and am writing this in C, so i'm kind of "iffy" on the whole thing.

[Tim]

There's too much to explain on a board like this. Try browsing through the Cosmos source code:
http://www.execpc.com/~geezer/os/